TL;DR
GPT-5.5 is an external OpenAI model. It is OpenAI's April 2026 refresh of the GPT-5.4 line — same 1.05M-token context, same price, but better reasoning, better tool reliability, and noticeably stronger recall on documents that push past 500K tokens. If you've been defaulting to GPT-5.4, switch.
Open Oakgen Agent Chat to research, plan, and create with Oakgen Auto, Oakgen Max, or Oakgen Pro Max. Creative generation uses the same credit balance.
What's actually new
GPT-5.5 keeps the architectural footprint of GPT-5.4 (dense transformer, multimodal in/out, native reasoning tokens) but ships three concrete improvements:
- Reasoning depth. Internal reasoning chains are longer and the model spends them more wisely. On math, code, and multi-step planning prompts, the difference shows up as fewer "I should double-check" hedges and more correct first answers.
- Long-context recall. GPT-5.4's recall above 500K tokens was patchy — names, numbers, and one-off facts got dropped. GPT-5.5 retrieves them reliably across the full 1.05M window.
- Tool-use reliability. Function-calling failures (malformed JSON, hallucinated tool names) drop noticeably. For agent stacks, that translates into fewer retry loops.
What did not change: pricing, max output tokens, vision support, or the OpenRouter model contract. It is a drop-in replacement at the API level.
GPT-5.5 vs GPT-5.4 head-to-head
| Capability | GPT-5.5 | GPT-5.4 | Winner |
|---|---|---|---|
| Context window | 1,050,000 | 1,050,000 | Tied |
| Max output tokens | 128,000 | 128,000 | Tied |
| Native reasoning tokens | Yes (deeper) | Yes | GPT-5.5 |
| Long-context recall (>500K) | Strong | Patchy | GPT-5.5 |
| Tool / function calling | Higher reliability | Solid | GPT-5.5 |
| Vision input | Yes | Yes | Tied |
| Input price / 1M tokens | $2.50 | $2.50 | Tied |
| Output price / 1M tokens | $15.00 | $15.00 | Tied |
| Cached input / 1M tokens | $0.25 | $0.25 | Tied |
Same money, more capability. That's the whole pitch.
When to use GPT-5.5
Long documents. If you regularly paste contracts, codebases, or research papers above 500K tokens, the recall improvement alone is worth the switch. GPT-5.4 would lose threads; GPT-5.5 keeps them.
Agent loops. Tool-use reliability compounds in multi-turn agents. A 2% reduction in malformed tool calls can mean 30% fewer end-to-end failures over a 10-step plan.
Reasoning-heavy chat. Math, code reviews, debugging sessions, technical writing edits — anywhere you'd previously have used GPT-5 (Thinking) or GPT-5.4 with high reasoning effort, GPT-5.5 narrows that gap and ships answers faster.
Vision + reasoning combined. Paste a chart or screenshot and ask a multi-step question. GPT-5.5's joint vision-and-reasoning trace is meaningfully tighter than 5.4's.
When GPT-5.4 (or another model) still makes sense
- Reproducibility. If you've pinned evals or production prompts to GPT-5.4's exact outputs, don't churn until you re-run your evals on 5.5.
- Cost-sensitive bulk text. For pure text reasoning at scale, Deepseek V4 Pro is still 60–80% cheaper. See our Deepseek V4 Pro vs GPT-5 comparison for the breakdown.
- Frontier vision. For pure visual reasoning (charts, screenshots), Claude Opus 4.7 and Gemini 3.1 Pro are competitive — try all three on your task.
Using GPT-5.5 via OpenRouter
The model ID is openai/gpt-5.5. A minimal streaming call looks like this:
import OpenAI from "openai";
const client = new OpenAI({
apiKey: process.env.OPENROUTER_API_KEY,
baseURL: "https://openrouter.ai/api/v1",
defaultHeaders: {
"HTTP-Referer": "https://oakgen.ai",
"X-Title": "Oakgen.ai",
},
});
const stream = await client.chat.completions.create({
model: "openai/gpt-5.5",
messages: [{ role: "user", content: "How many r's are in 'strawberry'?" }],
stream: true,
});
for await (const chunk of stream) {
const delta = chunk.choices[0]?.delta?.content;
if (delta) process.stdout.write(delta);
if (chunk.usage) {
console.log("\nReasoning tokens:", chunk.usage.completion_tokens_details?.reasoning_tokens);
}
}
Open Oakgen Agent Chat to research, plan, and create with Oakgen Auto, Oakgen Max, or Oakgen Pro Max. Creative generation uses the same credit balance.
Try it on your own prompt
Oakgen Agent Chat offers Oakgen Auto, Oakgen Max, and Oakgen Pro Max. Test the external models discussed here in their providers' products; use Oakgen for research, planning, and creative production.
For the wider LLM landscape, see our ChatGPT vs Claude vs Gemini vs Grok 2026 comparison.
Frequently asked questions
What is GPT-5.5 and how is it different from GPT-5.4? GPT-5.5 is the April 2026 successor. Same context window, same price, deeper reasoning, better long-context recall, and more reliable tool calling.
Is GPT-5.5 available in Oakgen Agent Chat? Oakgen Agent Chat offers Oakgen Auto, Oakgen Max, and Oakgen Pro Max. The external models compared in this article are not available as named choices in that selector.
Does GPT-5.5 support vision and tool use? Yes. Vision input, function calling, structured outputs, reasoning tokens, response_format, and seed are all supported.
What is the context window of GPT-5.5? 1,050,000 input tokens with up to 128,000 output tokens.
Should I switch from GPT-5.4 to GPT-5.5? For most users, yes — the price is the same and the model is better. Only stay if you have pinned evals against GPT-5.4's exact behavior.
Frequently asked questions
Can I use GPT-5.5 with the OpenRouter SDK?
Yes. Use model ID 'openai/gpt-5.5' with the OpenRouter chat completions endpoint. Streaming is supported and reasoning tokens come through in the final usage chunk.
